As explained in the blog Begonias From Seed: For Experts Only, begonias are more difficult to grow from seed than almost any other annual. However, if you want to give it a try, here’s how.
What Kind of Begonia?
Unless you buy your begonia seed from a specialized source (the American Begonia Society, for example, offers a wide choice of unconventional begonias), you’ll most likely only encounter seed packs of two types of begonias: wax begonias (B. x semperflorens-cultorum) and related varieties, such as Benary begonias (B. x benariensis) and Dragon Wing begonias, and tuberous begonias, especially B. x tuberosa and B. boliviensis.
Of the two, wax begonias are the easiest to grow from seed. Tuberous begonias can be a bit challenging: they seem to falter if conditions aren’t just right.
The Right Light
The initial very slow growth of begonia seedlings means you really have to start them very early in the season if you want to use them in your garden the first year. Early February — or better yet January — are not too early in the Northern Hemisphere… and that poses a problem.
At that time of year, day lengths are still very short and the sun is very weak… not ideal for seedlings that need light to germinate and grow. As a result, germination under natural light can be sporadic to nil or if they do come up, they quickly falter. For that reason, it’s much better to grow begonia seedlings under a fluorescent lamp where you can control the light quality and day length.
Tuberous begonias are even more tricky because they tend to go straight into producing tubers under short days (less than 12 hours) rather than producing more leaves that will lead to the flowering plant you want. Yet more than 15 hours or more doesn’t contribute to either growth or flowering. That’s why it’s best to use a timer to control day length, setting it at 14-hour days. That will ensure bigger plants in spring and more flowers throughout the summer.
Wax begonias are not as affected by day length as tuberous begonias, but still grow and bloom more abundantly when grown under 14-hour days.
How to Sow Begonias
These days most begonia seed packs contain pelleted seed. This makes them easier to handle and space equally, but pelleted seeds do cost much more. Often you have no choice, though, as many begonias are only available as pelleted seed.
To sow begonia seeds, fill a pot or tray of fine potting mix (a seed blend would be perfect) and moisten it well, allowing any surplus water to drain away. The soil should be evenly moist, but not soggy. Level the soil: you don’t want any dips or depressions.
Now place the seeds evenly on the surface of the mix. If they aren’t pelleted, it will be hard to sprinkle them evenly, but do your best. Firm them lightly with a block of wood to make sure they’re in contact with the growing mix, but don’t cover them with soil: they need light to germinate.
Next spray the surface with lukewarm water and place the seed container in a clear plastic bag or mini-greenhouse to ensure the high humidity necessary for good germination.
Move the container to a spot about 6 to 12 inches (15-30 cm) below a fluorescent lamp. Its gentle warmth will be beneficial for germination and will help keep the container warm. It should remain above 70˚F (21˚C) even at night.
In 7 to 14 days, sometimes a bit longer, you should see very tiny green leaves appear. It will look like the soil is covered with green moss at first: you’ll need a magnifying glass if you want to pick out individual details. When the leaves are large enough to make out them out clearly without a magnifying glass, remove the covering to increase air circulation.
Maintain good lighting and water when the soil starts to dry out: it should remain slightly moist at all times. You may find it easier to pour tepid water into the tray below and let the soil “drink” its fill of moisture rather than watering from above with a watering can whose stream can knock the fragile seedlings over.
When the seedlings have two or three leaves, move them into individual small pots or plug trays. Since they are still very small and fragile at this stage, it may be wise to cover them with a clear plastic dome or bag again for a few days until they recover from any transplant shock.
Afterwards, as the seedlings grow, keep the soil slightly moist and fertilize occasionally with a dilute soluble fertilizer.
When the seedlings have 5 to 6 leaves, you’ll find they begin to grow more quickly and with greater vigor. They are now “well established” and better capable of facing variations in temperature, watering, etc. In fact, by this stage, they’ll be as robust as any other seedling and won’t need as much coddling.
Even so, always keep the soil at least slightly moist at all times: it should never dry out completely, nor should it soak in water for hours on end.
When the outside temperatures have warmed up and nights remain above 50˚F (10˚C), you can begin acclimatizing the seedlings to outdoor conditions, then planting them in the garden. Most begonias prefer partial shade, but some can tolerate full sun or shade. With a minimum of care, they should bloom all summer.
If you’re reached the “planting out” stage with your begonia seedlings, congratulations: you have succeeded where many other gardeners have failed. Give yourself a big pat on the back!